η6-Arene-Zirconium-PNP-Pincer Complexes: Mechanism of Their Hydrogenolytic Formation and Their Reactivity as Zirconium(II) Synthons.

The cyclometalated monobenzyl complexes [(Cbzdiphos^R-CH)ZrBnX] 1 ^{i Pr} Cl and 1 ^Ph I reacted with dihydrogen (10 bar) to yield the η⁶-toluene complexes [(Cbzdiphos^R)Zr(η⁶-tol)X] 2 ^{i Pr} Cl and 2 ^Ph I (cbzdiphos=1,8-bis(phosphino)-3,6-di- tert-butyl-9 H-carbazole). The arene complexes were also found to be directly accessible from the triiodide [(Cbzdiphos^Ph)ZrI₃] through an in situ reaction with a dibenzylmagnesium reagent and subsequent hydrogenolysis, as exemplified for the η⁶-mesitylene complex [(Cbzdiphos^Ph)Zr(η⁶-mes)I] ( 3 ^Ph I). The tolyl-ring in 2 ^{i Pr} Cl adopts a puckered arrangement (fold angle 23.3°) indicating significant arene-1,4-diido character. Deuterium labeling experiments were consistent with an intramolecular reaction sequence after the initial hydrogenolysis of a Zr−C bond by a σ-bond metathesis. A DFT study of the reaction sequence indicates that hydrogenolysis by σ-bond metathesis first occurs at the cyclometalated ancillary ligand giving a hydrido-benzyl intermediate, which subsequently reductively eliminates toluene that then coordinates to the Zr atom as the reduced arene ligand. Complex 2 ^Ph I was reacted with 2,6-diisopropylphenyl isocyanide giving the deep blue, diamagnetic Zr^II-diisocyanide complex [(Cbzdiphos^Ph)Zr(CNDipp)₂I] ( 4 ^Ph I). DFT modeling of 4 ^Ph I demonstrated that the HOMO of the complex is primarily located as a 'lone pair on zirconium', with some degree of back-bonding into the C≡N π* bond, and the complex is thus most appropriately described as a zirconium(II) species. Reaction of 2 ^Ph I with trimethylsilylazide (N₃TMS) and 2 ^{i Pr} Cl with 1-azidoadamantane (N₃Ad) resulted in the formation of the imido complexes [(Cbzdiphos^R)Zr=NR′(X)] 5 ^{i Pr} Cl-NAd and 5 ^Ph I-NTMS, respectively. Reaction of 2 ^{i Pr} Cl with azobenzene led to N−N bond scission giving 6 ^{i Pr} Cl, in which one of the NPh-fragments is coupled with the carbazole nitrogen to form a central η²-bonded hydrazide(−1), whereas the other NPh-fragment binds to zirconium acting as an imido-ligand. Finally, addition of pyridine to 2 ^{i Pr} Cl yielded the dark purple complex [(Cbzdiphos ^iPr)Zr(bpy)Cl] ( 7 ^{i Pr} Cl) through a combination of CH-activation and C−C-coupling. The structural data and UV/Vis spectroscopic properties of 7 ^{i Pr} Cl indicate that the bpy (bipyridine) may be regarded as a (dianionic) diamido-type ligand. [ABSTRACT FROM AUTHOR]